Field of the Invention
The present invention relates to an apparatus including a tank having a tank bottom and a delivery unit for a liquid, wherein the liquid is, in particular, a liquid reducing agent for exhaust-gas aftertreatment.
In order to remove nitrogen oxides (NOx) in the exhaust-gas flow of internal combustion engines, a liquid reducing agent is preferably injected into the exhaust-gas flow in order to convert the nitrogen oxides present in the exhaust-gas flow into elementary nitrogen (N2) and water (H2O) through the use of a catalytic converter. An active substance, for example ammonia (NH3) and/or urea (CH4N2O), is preferably dissolved in water and added as a liquid reducing agent. In order to store the liquid reducing agent, a tank is provided which interacts with a delivery unit so that it is possible to deliver the reducing agent to the exhaust-gas flow from the tank.
A number of technical problems are associated with the delivery and storage of the reducing agent, in order to supply a predefined quantity of the reducing agent to the exhaust-gas flow under all operating conditions. Those problems are based, in particular, on the fact that the liquid reducing agent, in particular the aqueous urea solution, can freeze. In order to prevent the liquid reducing agent from freezing, it is possible to add, for example, antifreeze, with the result that the freezing point of the aqueous urea solution, which is usually about −11° C., is lowered to −40° C.
Even when such an antifreeze or freezing-point depressant is used, it must nevertheless be ensured that the nitrogen oxides in the exhaust-gas flow are reduced even at very low temperatures in the vicinity of the motor vehicle. For that purpose, it may be necessary first of all to thaw or melt the reducing agent. To that end, various configurations for heating the tank and/or a partial volume of the tank have already been proposed. However, those configurations are to some extent not suitable for carrying out repeated thawing and freezing in such a way that liquid reducing agent can be added reliably to the exhaust-gas flow.
Particular difficulties arise when the liquid reducing agent is extracted close to the tank bottom and through the tank bottom. In that case, it has been found, for example, that although the frozen reducing agent can be thawed through the use of a heater on the tank bottom, regions of the frozen liquid that are located thereabove cannot be reached, and therefore a thick shell of ice remains around the removal point. However, the extraction of the liquefied reducing agent by suction results in a vacuum, against which the pumps that are usually employed in that case cannot work.